This invention relates to a test structure that is used in testing the rotational speed of each drive member on opposite sides of a crawler tractor. This invention also relates to a method of testing of the speeds of rotating drive members on the crawler tractor and for adjusting the respective speeds.
In many crawler type tractors there are provided independent transmissions that drive the respective drive sprockets for the tracks on the tractor. In such arrangements, there is normally a single control for determining the engine speed. The problem that exists is that, the respective transmissions may create small variations in speed of the drive sprockets for the tracks. Thus, one side of the tractor will move at a different rate, even though a slight amount, than the other side of the tractor. For example, if the right drive sprocket is going faster, then the unit will move or mis-track to the left over a distance, conversely, if the left rack is going faster the unit will mis-track or move to the right. There are normally provided in the respective transmissions adjustments for providing small increases or decreases of speed which eventually is translated to the respective sprockets.
In many such crawler tractors, the manufacturer sets up certain limits or standards that each tractor may mis-track. For example, a manufacturer may limit the amount of mis-tracking to two feet in a one hundred foot distance. Heretofore, it has been necessary to drive the tractor over this distance and determine whether a mis-track outside this limit was occurring. This was not only time consuming and expensive, there were also questions concerning the accuracy of such systems.
With the above in mind, it is a primary purpose of the present invention to provide a test fixture which will measure the potential mis-tracking on a crawler tractor. It is also a purpose of the present invention to provide a method of testing, recording and correcting the speed of the crawler tractor drive sprockets so that the difference in speed between them fall within acceptable limits.
It is a further purpose of the present invention to utilize a system for determining the speed of the sprockets or their rotary drive members on a tractor in which each sprocket or rotary dive member is driven by its own transmission. The transmission is a hydrostatic drive that includes a pump and a motor with the pump driving the motor and the motor driving a shaft that through suitable drive means rotates the drive sprocket. There are adjustments in both the pump and motor in each transmission to adjust the speed of the sprocket.
In actual operation of such a tractor, when the tractor starts at zero ground speed there is full displacement of the motor and zero displacement of the pump. As the speed increases the pump displacement increases towards its maximum. After reaching its maximum, the motor displacement decreases. Therefore, it becomes apparent that the selection of adjustment in the pump and motor of the hydraulic transmission will depend upon the ground speed of the tractor. For example, if the tractor would mis-track only at high ground speed, the motor portion of the transmission should be adjusted to eliminate this mis-tracking. Similarly should the tractor mis-track only at minor speeds, the pump portion of the transmission would be adjusted to vary the speed. It is for this reason that the present test device must be capable of determining mis-tracking at various speeds.
Also effecting mis-tracking is the "ground coupling" affect between the two tracks of a track laying tractor. It has been determined that the lesser the resistance to turning by the ground, the more mis-track will be observed. Lower resistance is generally associated with softer or looser soil. It is therefore necessary in a test fixture or stand to simulate the "ground coupling" affect. It is therefore an additional purpose of the present invention to simulate the "ground coupling" affect through a drive that includes a pair of axles or shafts that are on the stand in parallel relation to the axis of the tractor rotary drive members and which have a drive, preferably of a chain type nature between the tractor and the test axles. The inner ends of the test axles are joined by a differential type of transmission which connects to a central and singular driven shaft. The driven axles, being fully journaled, allows very minimal resistance to turning so that they simulate the potential worse ground condition for mis-tracking which is loose soil.
In actual operation the weight and the drive characteristics of the tractor affects the amount of mis-tracking. For each model or size of tractor these will vary. In order to simulate the effect of weight and tractor characteristics, the driven central shaft referred to above is connected to a dynamometor which affords load on or resistance to rotation of the driven shaft and which load or resistance may be varied in accordance with the weight and characteristics of the tractor that is being tested. Therefore, it is a further object of the present invention to simulate the weight and characteristics of the tractor through a variable resistance device, which in this particular instance is a dynamometor, to the driven shaft.
It is still a further object of the present invention to provide readout material for measuring the speed of the respective sprockets and rotary drive members so that it can be determined which of the hydraulic transmissions and which portion of the respective transmissions, i.e. the pump or the motor, should be adjusted to eliminate or lessen the mistracking of the tractor.